This invention relates to a support structure for superconducting magnets, particularly suitable for supporting the bucking magnet coils of an active magnetic shielding system.
As is well known, a magnet, if wound with wire possessing certain characteristics, can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnet coils to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of ten minutes) to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter "MRI").
Bucking magnet coils are auxiliary magnet coils provided proximate to the main magnet coils to shield the magnetic field produced by the main magnets, even in the presence of surrounding magnetic structures and/or magnetic flux emitting devices. The support structure for the main magnet coils is an annular cylindrical aluminum drum. The main coils are wound separately around stainless steel bobbins, placed in grooves machined in the drum, and are spaced axially along the inside of the drum. The bucking coils are annular coils spaced from, and surrounding, the main coils by a support structure secured to the drum, and carry currents in the direction opposite to that carried by the main coils to reduce the stray magnetic field outside the magnet. This is called active magnetic shielding.
However, in the process of energizing the magnets, or ramping the magnets to field, and in cooling the coils to superconducting temperatures, the coils are subjected to significant thermal and electromagnetic loading. As a result, actively shielded magnets pose difficult problems in terms of structural support. The principal reason for utilizing active magnets, as opposed to a passively shielded system, is that the latter would require massive amounts of magnetic material, such as iron, around the magnet, providing considerable increases in weight and volume. However, to minimize weight and volume through use of bucking coils, and thus realize the objectives of active magnetic shielding, it is important that the support structure for the bucking coils be relatively lightweight and yet withstand the significant magnetic and thermal loads placed upon it in an MRI. It is also important that the bucking coils maintain close positional accuracy, notwithstanding the significant thermal loads during initial operation or cooldown of the MRI, and notwithstanding the electromagnetic loads generated during energization and operation.
As a result, there are conflicting thermal, magnetic and mechanical considerations and factors which must be balanced and compromised to obtain an acceptable bucking coil assembly.